This invention relates to magnesium additives for iron and steel melts, and in this context magnesium is intended to include alloys consisting predominantly of magnesium.
In addition to its use as an alloy ingredient for metals, magnesium is also used in the treatment and refinement of metals and alloys. For example, it is known that the addition of magnesium to molten iron such as is delivered from a blast furnace results in a form of iron, frequently termed nodular or spheroidal graphite iron, which is more ductile than cast iron. Further examples include the addition of magnesium to certain iron melts to produce a malleable iron or at least an iron which can be converted to a malleable iron by an annealing process much less protracted than would normally be required and in the desulphurisation of certain steels. The removal of sulphur by magnesium proceeds more rapidly than removal by any other reagents, such as sodium carbonate and calcium carbide.
There are, however, problems in adding magnesium to molten iron and steel. Magnesium has a boiling point of about 1120.degree.C whereas iron and steel have melting points of about 1400.degree.C and 1500.degree.C, respectively. Thus, not only does the magnesium when introduced into molten iron or steel tend to evaporate before it can be absorbed into the molten metal but it tends to evaporate so vigorously that molten metal is spattered about and in some instances an explosion may occur. Also, molten iron and steel have higher densities than magnesium and exhibit high surface tension effects so that it is difficult to make the magnesium that has not evaporated penetrate into the molten metal and become homogeneously distributed throughout the final cast product.
Various ways of overcoming these problems have been proposed and include placing the magnesium at the bottom of a receptacle and pouring the molten iron of steel on to it, or taking a quantity of molten iron or steel and introducing magnesium particulate form into the melt for example, by spreading the particles over the melt and stirring it vigorously, or by projecting the particles at the surface of the melt or by passing the particles through a tube of suitable material extending into the melt. Another way of introducing the magnesium into the melt that is finding increasing use is to introduce magnesium particles into the vortex created by pouring the melt through an annular orifice into a receptacle.
With all these ways the proportion of magnesium utilised in reaction with the iron or steel is only about 10 - 30%, the remainder being ejected from the melt as vapour with the result that there exists the dangers of spattering of molten metal and of explosion.
Attempts have been made to avoid these difficulties by impregnating a porous mass of carbon, such as coke, with magnesium and adding the mass to the ferrous melt. Such an impregnated mass has been made simply by immersing coke into molten magnesium. However, it is known that difficulties arise with this method of preparing such a material due to non uniformity of magnesium impregnation and contamination of the magnesium by, for instance, oxidation products during impregnation. Therefore the release of magnesium from such a material as vapour in a ferrous melt is still far from uniform and can give variable results.